pass-through: adjust pIRQ migration

pass-through: adjust pIRQ migration

For one it is quite pointless to iterate over all pIRQ-s the domain has
when just one is being adjusted. Introduce hvm_migrate_pirq() as an
externally accessible function.

Additionally it is bogus to migrate the pIRQ to a vCPU different from
the one the event is supposed to be posted to - if anything, it might be
worth considering not to migrate the pIRQ at all in the posting case.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>
Acked-by: Andrew Cooper <andrew.cooper3@citrix.com>